Now that we've established the basic framework for all future study, a word needs to be said about the value and limitations of the roles assigned to the players.

Paired practice forms the basis for most of our study. Solo practice is certainly valuable, particularly for the very early lessons. Further, the same principles established for two-person practice can be applied to multiple-person encounters. But unless otherwise specified, we will continue to assume a practice oriented toward two people working together to solve certain problems. in group training, partners should mix freely to form new pairs so as to optimize experience with a variety of body and personality types.

For most practice, partners agree to certain roles. They should swap roles frequently, but only at agreed upon intervals.

As is discussed in the section on Theory, this system assumes a problem posed by the Collision of Solids, and a possible resolution through Emptiness. Accordingly, one person explores the creation of conflict by closing distance and increasing pressure (compression, tension, and torque), while the other person explores the potential for resolution through joining with the Solid and orienting the system towards Emptiness.

Students of traditional aikido practice will immediately try to map these roles onto the standard division of uke and tori (or nage). While there is considerable overlap with traditional methods and the current one under discussion, correspondences only go so far. This practice is spontaneous and improvisational, and within the bounds of safety and respect, anything can happen. What one person is attempting is not inherently more or less aiki than what the other one is doing, and it's possible for either role to prevail.

For convenience, I call these roles Solid and Empty. One person must persistently "push solid," while the other adheres to an agenda of "pushing empty." Together, they are exploring the circumstances under which Solid is the more sensible strategy, or when Empty is more optimal.

Before we go any further, let's remember that all training is artifice. Correct artifice makes training more realistic, but it is still artifice. The designated role-play of Solid and Empty is such an artifice. It is not intended to simulate the reality of combat. Rather, it focuses the significant variables of an encounter and makes it possible to examine what is happening at each moment.

Inevitably, one player or the other will see an opportunity to take advantage of their partner by momentarily switching roles. In most regular practice, this is to be avoided as much as possible. Keep in mind that there will always be small ambiguities (Solid must travel through empty space in order close distance, Empty must apply light pressure to join with Solid, etc.), but clear changes in protocol are noticeable to anyone paying attention. If, for example, the person playing Empty has the positional and structural advantage and throws or pins their Solid counterpart by the increase of pressure, we should say that Solid has "won" the moment. Conversely, if Solid unbalances Empty by way of a momentary creation of a vacancy, then we should say that Empty prevailed.

What must be understood is that although there is a kind of competition taking place, it is more between the strategy of the Solid versus the Empty than between the players themselves. True, if they are doing their job right, they are staunchly implementing the behavior of the specific archetype. Even so, the point of the study is to see whether Solid or Empty, as principles rather than people, has prevailed.

If players were able to be perfect advocates at all times for their assigned parts, this point would be unnecessary. However, constant perfection is never available, or else practice would be needless. Switching roles in the moment without notice, is generally to be avoided. With more practice, players will become more comfortable in adhering to their assigned function. When temptation is strong and roles are wrongly switched, everyone should simply note the outcome and assess whether it was truly the principle of Solid or the principle of Empty that prevailed, regardless of which player executed the action. This is not an opportunity for scolding or accusations of cheating -- it's all learning, and it's all valuable if participants are sincerely trying to work within the framework.

The reasons for the separation of roles are simple. There is always increased risk when both players are pushing solid at the same time. This is exactly the thing that increases the likelihood of the Collision of Solids with its concomitant chance for violence or harm. Efficiency is decreased even when the outcome is otherwise successful. It also tends to reinforce the players' impulse to compete with one another rather than collaborate in the competition of Principles. To "collaborate in competition" may seem paradoxical, but with careful reflection it makes perfect sense, and cooperative but challenging training manifests its value.

Now, having said all this, there comes a time when advanced players will want to abandon the strictures of preassigned roles. This is to be encouraged, but only when everyone involved is adept at both respective parts of Solid and Empty and have demonstrated the mental discipline required to stay within the bounds of constrained play. Full freedom play is tremendous fun when the partners are skilled and mutual trust and confidence is present. It should be agreed at the outset and understood by all players that this is the current mode for learning and exploration.

In this freestyle training, either player is able to initiate the encounter by closing distance. Either player can choose to increase pressure. Either player can release pressure by pushing empty. What remains consistent in both constrained and free play is that Solid against Solid should be avoided. To repeat, it is less safe, less efficient, and less productive. The trick is to learn to advantageously release the pressure as it builds just enough energy such that fluid motion can be channeled.

At this advanced level of play, we see that the Solid and Empty comprise a surprising oneness. Pushing solid to overcome obstruction and break through to freedom is only natural, and one can learn to do so mindfully, proportionately, and safely. Pushing empty to bide time and await the fullness of the moment in order to strike decisively is also natural. The clarity that comes with spacious thinking and perception helps us to know when and how best to act.

Free play is a purer kind of improvisation. It is not, however, the fruition of our other training or a higher expression of our understanding. Rather, it is simply another modality to explore alongside our other methods. To be done well, it requires some advanced understanding and skills, and a certain familiarity with the system, but true advanced training is simply more thorough immersion in the basics.

This business of the Solid and the Empty is more or less another way to convey Doing versus Non-Doing. To survive, we must express our will on the world. We make changes, we rearrange matter, expend energy, increase pressure, move minds. This Doing is the lesson of Solid, and part of our understanding is that whenever we commit ourselves to forceful action, we risk some measure of violence. The present course of study is one way to make all our doing increasingly aiki, where cause fits effect in greater accord.

Survival also requires us to manage things being done to us. Our will to endure, to remain unperturbed in the face of agitation, to recover our zones and continue with a normalized integrity is the Non-Doing lesson of Empty. It is directed non-doing that allows systems to become self-correcting.